1. Field of the Invention
The present invention particularly relates to a power steering controller for a vehicle that achieves an appropriate steering feel at an entrance to a curve.
2. Description of the Related Art
For a vehicle, steering support devices for providing steering support by generating steering torque in various manners with a power steering motor have been suggested and put into practice. For example, Japanese Unexamined Patent Application Publication (JP-A) No. 2008-44531 discloses a technique of a steering support device which calculates a steering target value on the basis of the shape of a driving road on which a vehicle runs, and provides steering support in accordance with the steering target value, and a determination is made as to whether the vehicle runs at an entrance to a curve of the driving road. When the vehicle is determined to be running at the entrance to the curve, the steering support device increases the steering target value, and suppresses delay of additionally turning the steering wheel at the entrance to the curve, as compared with a case where the vehicle runs at a place other than the entrance to the curve.
In general, in a vehicle steering system, there is a dead zone between an actual wheels angle and a steering wheel angle of the steering wheel when the steering wheel is around the central position because of friction and the like that are set in view of reverse input, vibration, and the like which are given by the tires or the configuration of mechanical parts between the steering wheel and the steered wheels. In the dead zone, the position where the steering wheel angle based on the steering wheel actually exists may change relatively due to disturbance and the like given by the road surface, and therefore, the driver cannot recognize the actual position until the driver starts steering. For example, when the driver turns the steering wheel to the left by a certain angle to make a left turn, the driver may feel the response during steering is not enough and tends to turn the steering wheel too much if the steering wheel angle of the steering wheel exists at the right side of the dead zone. As described above, when the driver turns the steering wheel too much, it is necessary to relieve the force in the opposite direction in order to correct the steering angle. In contrast, when the steering wheel angle based on the steering wheel exists at the left side of the dead zone, the driver tends to turn the steering wheel insufficiently during steering on the contrary. In order to improve the problem of the difference in the steering feel due to the dead zone existing in such steering system, the rigidity of the joint may be improved, and the gear meshing backlash may be reduced, but this is trade-off for performance of the steering system such as vibration and noise. It may be possible to add, e.g., steering returning control, but this may cause a new problem such as ensuring continuity of change of steering force during steering over the central position. The steering support control disclosed in JP-A No. 2008-44531 explained above is to suppress the delay of additionally turning the steering wheel at the entrance to a curve in the lane keep control, and is unable to solve awkwardness in the steering feel caused by the existence of the dead zone of the steering system at the entrance to the curve as described above.